Researchers from the Centers for Disease Control and Prevention announced the discovery of a new tick-borne virus this week, one of several newly identified pathogens that ticks are passing to humans. Why are so many new tick-borne diseases being reported?

Improvements in medical technology, mostly. The newly discovered tick-borne diseases have probably been infecting humans for years, but they’re not easy to spot. Many victims never realize a tick has bitten them, and the symptoms, such as fevers, aches, and fatigue, are not distinctive and mirror those of common summer viral infections. The patient’s immune system usually fights off the illness unassisted, so doctors don’t bother running the battery of tests required to identify a new pathogen. Only in the rare cases when a patient struggles with the infection are physicians likely to make a genuine discovery. (The disease identified this week, a member of the phlebovirus family, hospitalized two men in Missouri.) They order blood smears or antibody tests to identify the presence of a pathogen, and genetic analyses—which were either inefficient or completely unavailable to doctors just a few years ago—alert researchers to the presence of the previously unidentified bug. Researchers have also become more aggressive recently, with some searching within ticks themselves for evidence of new pathogens.

Lyme disease is a classic example of how long a disease can exist in a population without being identified. Genetic research has suggested that the pathogen responsible for Lyme disease has afflicted humans for more than 5,000 years. Examinations of a mouse specimen collected from Central Park in the late 1800s suggest that the disease has been present in the United States for more than a century, and case reports indicate that Europeans were carrying the disease at about the same time. In the early 1970s, fishermen in Long Island complained of a form of arthritis they called “Montauk knee,” but it wasn’t until later in the decade that doctors figured out that it was caused by an infectious agent. In 1982, researchers finally identified the spirochete that carried the illness. And Lyme disease, with its telltale bull’s-eye rash, is usually easier to identify than many other tick-borne illnesses.

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Environmental change has also played a role in the recent discoveries. Americans spent centuries converting the Northeastern woodlands into farms, hunting or driving out much of the local fauna. In the late 19th and 20th centuries, however, agriculture moved west, and many farms turned back into fragmented forests in close contact with human settlement. Animals that survive best in so-called “edge habitats,” like deer and mice, came to dominate the region. Dense populations of those animals made fertile breeding ground for ticks and the pathogens that love them.

Increased tick density plays two roles in the discovery of tick-borne diseases. First, and probably more significantly, it leads to more human infections and gives doctors more opportunities to identify existing pathogens. It’s also possible, although largely unproven, that tick-borne pathogens are evolving more quickly than in the past. Ticks are expanding their ranges, bringing different species of the pests into contact with one another, sometimes as they ride on the same deer, dog, or rodent. This may result in microbes jumping between tick species. As pathogens are exposed to new environments, they may evolve into subtly different organisms, some of which might be more dangerous to human hosts.

It should be noted that humans are dead-ends for tick-borne pathogens—either they kill us or we kill them long before they have the chance to jump to the next host. So ticks don’t really evolve inside the human body. Rather, it’s the increased density of animal hosts like deer and mice that create new opportunities for the evolution of tick-borne pathogens. If those changes happen to afflict humans, we are little more than collateral damage.

Explainer thanks Brian Allan of the University of Illinois at Urbana-Champaign, Carl Brenner of the National Research Fund for Tick-Borne Diseases, and Peter Krause of the Yale School of Public Health.